The present invention concerns seals for high temperature applications, tools having such seals, and forming processes carried out in sealed cavities.
Metal articles are often forming at elevated temperature at which the metal is more ductile. xe2x80x9cQuick plastic formingxe2x80x9d and xe2x80x9csuperplastic formingxe2x80x9d refer to processes for forming metallic materials such as aluminum alloys of aluminum, magnesium, and titanium at temperatures at which the materials have exceptional ductility. Metal alloys suitable for superplastic forming generally have tensile ductility ranging from 200% to 1000%. For example, certain aluminum alloys such as SP Aluminum Alloy 5083 can be deformed with air at temperatures of about 450 to about 600xc2x0 C., depending on the specific composition, to take the shape of a die surface inside of one-half of a sealed tool cavity. C. H. Hamilton and A. K. Ghosh, xe2x80x9cSuperplastic Sheet Forming,xe2x80x9d Metals Handbook, Ninth Edition, Vol. 14, pages 852-868, incorporated herein by reference, provides a background description of practical superplastic metal alloys and SPF processes. The authors describe several superplastic aluminum and titanium alloys that are suitably fine grained for SPF processes.
Stretch forming is one SPF process that is adaptable to forming relatively large sheets of superplastic metal, e.g. superplastic aluminum alloys, into automobile body panels or other large parts. In stretch forming a flat sheet blank is gripped or clamped at its edges by two complementary tool parts that, when closed together, meet around a periphery of an inner, forming volume within which the sheet is held. The tool parts are sealingly closed together and the sheet is heated to its SPF temperature. One of the tool parts has a cavity with inner shaping surface [xe2x80x9cdie surfacexe2x80x9d] opposite one face of the sheet. The second tool part, opposite the other face of the sheet, forms a pressure chamber, with the sheet as one wall, to contain the working gas for the forming step. The tool parts and the sheet are maintained at an appropriate forming temperature, for example by using electric resistance heating elements located in press platens or embedded in ceramic or metal pressure plates located between the parts and the platens. The chamber formed by the second tool part is pressurized using a suitable gas such as air or argon. The central, unclasped portion of the heated sheet is forced by the pressure to stretch and plastically deform into conformity with the die surface to make an article of the desired shape. The rate of pressurization is controlled so the strain rates induced in the sheet being deformed are consistent with the required elongation for part forming. Suitable strain rates are usually 0.0001 to 0.01 sxe2x88x921. The part is then cooled and removed from the tool. Vehicle body panels and other articles of complex shape may be formed by this process.
Rashid et al., U.S. Pat. No. 6,253,588, incorporated herein by reference, describes another representative superplastic forming process, quick plastic forming, using a magnesium-containing aluminum alloy having a particular microstructure. The superplastic forming (SPF) process is usually a relatively slow, controlled deformation process that yields complicated products. The quick plastic forming (QPF) process is a variant of SPF in which a higher strain rate allows a part to be formed in a much shorter time. An advantage of SPF and QPF processes is that they often permit the manufacture of large single articles that cannot be made by other processes such as conventional sheet metal stamping. Sometimes a single SPF part can replace an assembly of several parts made from non-SPF materials and processes.
When the periphery of the sheet is held in a fixed position between binding surfaces of the two complementary tool parts (i.e., the edges of the tool parts that contact the sheet) for an SPF process, the binding surfaces grip the sheet in a gas tight seal to allow for pressurization on one side of the sheet. The sheet does not flow over the binding surface as is typical in a conventional deep drawing operation. It is common to use a raised seal bead to grip the periphery of the sheet. For instance, male rectangular cross-section beads may be employed on one tool surface while the opposing binding surface is flat. A typical seal bead has a raised rectangular or trapezoidal cross-section approximately 10-15 millimeters wide and 0.5-1 mm tall. Because the tool parts must be well sealed in the superplastic forming process to achieve the necessary pressure to force the metallic material against the die, it would be desirable have a seal bead that forms as gas-tight of a seal as possible during use of the tool. If the tool is not adequately sealed, the article may not be fully formed. One problem encountered in superplastic forming is poor sealing due to slight misalignment of the tool parts or inadequate clamping pressures. It would thus be desirable to provide a seal bead that can be sealed with less pressure and/or can form a good seal even if the complementary tool parts are slightly misaligned.
Another problem encountered in superplastic forming is sticking of the formed sheet to the tool in the vicinity of the seal bead during part extraction. Because the sheet components are very deformable at the forming temperature, sticking can distort the panel during panel extraction due to uneven forces that may be applied in dislodging the part. Distortion is undesirable when a class A surface or dimensionally accurate part is required. Sticking also excludes effective use of robots for handling the parts during production because the amount of sticking is not predictable and individualized care must be taken in extraction.
The problem is particularly acute with aluminum sheet metal and severely slows the effective removal of an SPF-formed article from the binding portions of the tools. The aluminum sheet sticks primarily on the raised bead binding surface, but also may stick on the opposing flat binding surface. The sticking is due to reaction of the binding surfaces with freshly exposed, unoxidized aluminum at forming temperatures. This unoxidized, reactive aluminum is exposed at the sheet surface as a result of plastic deformation of the aluminum sheet during the clamping process prior to sheet forming. As the die is closed, aluminum is extruded (locally) away from the volume clamped between the bead and the opposing flat tool binding surface. As a result, the protective aluminum oxide film on the aluminum sheet surface is ruptured, and highly reactive aluminum is brought into intimate contact with the tool binding surface. The SPF forming tools are often made of, e.g., P20 steel, ductile cast iron or tool steel. For most such tool materials, local reaction or microwelding occurs to locally bond the aluminum sheet to the tool and cause sticking and tearing during part removal. This sticking problem may be tolerable when low volume production articles can be carefully pried from the tool, but the problem cannot be tolerated when high production rates are required. To adapt SPF to the production of automotive panels, practices must be developed that facilitate fast removal of an SPF-formed article from the forming tools.
One method that has been used to reduce sticking of the article to the toolsurfaces has been to apply a generous amount of a lubricant in the area of contact. This approach is not desirable for a number of reasons. First, applying a lubricant adds expense and preparation time to the forming process. Second, lubrication may need to be reapplied between parts. Additionally, the lubricant must be cleaned from the formed article and in some cases from the forming tool. These drawbacks make lubrication an unattractive solution. Another method that has been used to reduce sticking of the article to the tool has been to use a shallower seal bead to limit sheet deformation, as described by Scroth, U.S. Pat. No. 6,047,583, incorporated herein by reference. This reduces the surface area of unoxidized aluminum from the aluminum sheet that comes in contact with the tool. While this method does lead to less deformation, lubrication of the seal bead area is still needed to avoid deforming the part. Thus, it would be desirable to have a method of preventing a part from sticking to the seal bead that completely eliminates the use of a lubricant to produce a distortion free part.
The invention provides a tool for forming metal at elevated temperatures that has a binding surface with a porcelain enamel coating for holding the metal during forming. The binding surface contacts a metal sheet, for example, and may form a seal with the sheet. In general, the binding surface works with a complementary opposing binding surface of the tool in holding the metal item, e.g. metal sheet, and the first binding surface and the complementary opposing binding surface may together form a sealed perimeter about a portion of the sheet or other metal article. In one embodiment, a binding surface comprises a seal bead and the seal bead is provided with a porcelain enamel coating.
The invention further provides a method of forming a metal sheet at an elevated temperature in which the metal sheet is held in place by a binding surface of a tool. A binding surface in contact with the metal sheet has a porcelain enamel coating. The metal sheet is formed into a desired shape at an elevated temperature, preferably by a superplastic or quick plastic forming process. In one embodiment of the method, the first binding surface is used with a second, complementary and opposing binding surface, with the first and second binding surfaces together enclosing a portion of the sheet within a cavity of the tool and the binding surfaces defining a perimeter of the cavity. The opposing binding surfaces may form a sealed perimeter during heated forming of the sheet, for example by a superplastic forming process, and the first binding surface may have a seal bead with a porcelain enamel coating layer in contact with the metal sheet.
The enamel coated tool seal bead or other binding surface avoids sticking of the metal parts to the seal bead or binding surface. Further, in the quick plastic forming process or superplastic forming process, the enamel coating avoids problems caused by slight misalignment of the binding surfaces of the opposing tool halves. The enamel coating promotes an excellent seal without extreme clamping pressures. At hot forming temperatures, the porcelain enamel coating is softer than the die material, but still resistant to compressive stresses. Therefore, the porcelain enamel coated seal beads can correct small misalignments between the complementary tools than could uncoated seal beads. Moreover, in the case of a coated binding surface or seal bead, little or no lubricant is needed to avoid sticking. Finally, the enamel coating extends tool life by reducing or avoiding seal bead wear or binding surface wear.
xe2x80x9cAboutxe2x80x9d when applied to values indicates that the calculation or the measurement allows some slight imprecision in the value (with some approach to exactness in the value; approximately or reasonably close to the value; nearly). If, for some reason, the imprecision provided by xe2x80x9caboutxe2x80x9d is not otherwise understood in the art through this ordinary meaning, then xe2x80x9caboutxe2x80x9d as used herein indicates a possible variation of up to 5% in the value. All parts and percentages are by weight if not specifically indicated to be otherwise. xe2x80x9cAxe2x80x9d and xe2x80x9canxe2x80x9d are used to mean xe2x80x9cat least one;xe2x80x9d of the item is present; a plurality of such items may be present, when possible.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.